Currently high frequency linear wire antennas are commonly used in military transport in-service aircraft. Linear wire antennas have aerodynamic disadvantages and they also need extra auxilliary attachments to avoid possible safety risks caused by broken wires. Another drawback of wire antennas is that their mechanical and radio electrical characteristics are degraded during the aircraft service life due to vibrations caused by aerodynamic drag.
High frequency shunt antennas located in the vertical stabilizer of an aircraft are also known. Said antennas fail to efficiently cover lower frequencies due to their shorter length compared to wire antennas, as their length is limited by the available space inside the vertical stabilizer.
Shunt antennas have been used in aircraft vertical tail surfaces for many years. Their use in aircraft tail surfaces causes the whole tail surface to radiate/receive a high frequency radio signal and results in an almost equal 360-degrees propagation or ability to receive a radio frequency (RF) signal. The entire tail surface becomes a radiator/receiver of the RF signals from/to the antenna. The tail surfaces of the aircraft increase the surface area of the antenna and increase the propagation or ability to receive the RF signal to/from all directions.
An aircraft vertical tail comprises a leading edge, a torsion box, as its main supporting structure, and a trailing edge with control surfaces (rudders). The torsion box comprises a front spar, a rear spar and ribs extending from the front spar to the rear spar. Also, a known leading edge comprises several ribs, called leading edge ribs, attached to the front spar.
A shunt antenna for aircraft mountable in a dorsal fin of a vertical tail plane is disclosed in U.S. Pat. No. 7,511,624. An antenna radiating element is integrated into the dorsal fin structure being attached to the top inside composite skin of a new dorsal fin structure that replaces the original dorsal fin. The rear end of the antenna radiating element is connected to the fuselage such that a current loop is formed between the dorsal fin and the fuselage. A drawback of the disclosed invention is that a portion of the dorsal fin has to be replaced by a metallic part.
A dorsal high frequency antenna as that disclosed in U.S. Pat. No. 8,228,248 is also known. The antenna system is joined to the fuselage of the aircraft, so it is also mounted on the fuselage and it is electrically coupled to the surface of the vertical tail plane.
The above disclosed shunt antennas are mounted on the dorsal fin of the vertical tail plane and connected to the fuselage and tail surfaces which causes the external surface of the tail plane to radiate/receive.
Said shunt antennas have several drawbacks. They mainly interact with the surfaces covered by the dorsal fin, which limits the space available for them. For many aircraft, said limitation in size does not allow a correct operation at lower frequencies.
As they are not attached to the structure of the VTP, vibrations and deflections of the fuselage surfaces can degrade their electrical connections and therefore its radio electrical performances.
Additionally, some extra conducting elements are necessary to ensure grounding of the antenna to the primary structure of the VTP to drain high currents coming from a lightning strike.
Moreover, the situation of the antenna element near the surface of the dorsal fin makes it more exposed to be affected in case of a bird impact, the complete loss of the antenna being even possible.
Another known shunt antenna for an aircraft is disclosed in U.S. Pat. No. 8,354,968B1. The antenna is composed of a radiating element, that may be mounted on several placements on aircraft, such as inside fuselage, horizontal stabilizers or leading edge of vertical stabilizers. Furthermore its radiating element is composed of several shunt metallic plates put in parallel each other in order to decrease its reactance and so its parallel resistance. This configuration increase the total weight of the antenna assembly and in some aircraft locations its installation or integration may be very difficult since more space is required.